Bang! Pow! West 57th (8 p.m., WTVJ-Ch. 4, WTVX-Ch. 34), CBS-TV`s snazzy yuppie newsmagazine, unveiled last summer to a torrent of negative reviews, returns tonight for a second go-around. CBS wants to steal viewers away from ABC`s McGyver (no struggle there) and NBC`s Highway to Heaven (which real, live Americans - the kind who vote and buy groceries and firearms - actually watch). Tonight`s installment features a report on post-Duvalier police security in Haiti, an investigation into the purchase of machine guns by U.S. citizens, an interview with a 9-year-old boy growing up in a junkie-infested ghetto, and a profile of comedian Jay Leno. Correspondents are Bob Sirott, Jane Wallace, Meredith Vieira and John Ferrugia.